1. Field of the Invention
The present invention concerns a product consisting of a glass substrate carrying a thin conductive layer of metal oxide specifically exhibiting properties of low emission, low resistance and transparency. The invention also concerns a process for manufacturing such a product, particularly a process of pyrolysis of powders of metal compounds.
2. Background of the Prior Art
Windows intended for buildings are advantageously comprised of clear silica-soda-calcium glass that has high energy and light transmission factors, e.g. close to 90% for a thickness of 4 mm. To improve user comfort, especially in winter, by reducing energy loss due to leakage of calories from inside the building to the outside, windows are formed by covering one side of a sheet of glass with a conductive layer of so-called low-emission metal oxide, which increases the window's rate of reflection in infrared.
Such a window covered with such a layer can be combined with another sheet of uncovered glass, thereby trapping an air space between them, to comprise an insulating double window.
The low emission of the conductive layer is essential with a low resistance of the layer. Thus, sheets of glass having this type of layer can also be used as heating windows, especially in the automobile field, to form windshields and rear windows.
Windows carrying transparent conductive coverings and exhibiting properties of low emission and low resistance are known. They can be comprised, for example, of a glass substrate and a thin layer of metal oxide, such as a layer of tin oxide doped, for example, with fluorine or a layer of indium oxide doped with tin (ITO).
These layers can be obtained by different processes, processes under vacuum (thermal evaporation, cathodic pulverization, possibly with magnetron) or by pyrolysis of metal compounds in the form of a solution, powder or vapor projected onto the heated substrate. In this case, the compounds, on contact with the glass substrate heated to a temperature that is high but less than the glass-softening temperature, decompose and oxidize to form the metal oxide.
The layers of tin oxide doped with fluorine and the layers of ITO are advantageously obtained by pyrolysis of powders.
The layers of tin oxide doped with fluorine can be manufactured from powdered dibutyl tin oxide and gaseous anhydrous hydrofluoric acid, as described in French patent 2 380 997, and from dibutyl tin difluoride (DBTF) possibly mixed with DBTO as described in document EP-A-178 956 or EP-A-039 256. The layers of ITO can be obtained, for example, from indium formate and a tin compound such as DBTO as described in document EP-A-192 009.
Thus, it has been possible to obtain, by pyrolysis of powders, conductive layers of tin oxide doped with fluorine that have a thickness of 180 nm and have, at ambient temperature, an emission of 0.35 and a resistance of 10.10.sup.-4 .OMEGA.cm, approximately.
It has also been possible to manufacture, by pyrolysis of powders, layers of ITO with a thickness of 180 nm that have, at the ordinary temperature, emission of 0.11 and resistance of 2.10.sup.-4 .OMEGA..cm approximately.
Although these layers of tin oxide doped with fluorine exhibit worthwhile properties appropriate for certain uses, still other layers with lower emission are nevertheless being sought.
The layers of ITO have an emission lower than the emission of the layers of tin oxide doped with fluorine, but, to obtain this low emission, the layers of ITO must be submitted to a reducing thermal treatment, which increases the production cost of these layers.
Thus, we have sought to prepare products including a glass substrate carrying new conductive layers that are transparent and have, specifically, a low emission, possibly without necessitating any reducing thermal treatment.